US2262846A - Automatic audio tone control circuit - Google Patents

Description

Nov. '18, 1941. E. w. HEROLD AUTOMATIC AUDIO TONE CONTROL CIRCUIT Filed Sept. 15, 1939 AMPLIFIER 7'0 UTILIZATION MEAN-5' A UD/O KOLTAGE SOURCE OF I000 FREQUENCY (0 m1. E6

INVENTOR. EDWAR? [IE/20. .0

7K Ad-wb ATTORNEY.

Patented Nov. 18, 1941 Edward W. Herold, Verona, N. Y., assig'nor to Radio Corporation of America, a corporation of Delaware Application September 15, 1939, Serial No. 295,033

3 Claims.

My present invention relates generally to arrangements for automatically varying the audio frequency response characteristic of an audio transmission system, and more particularly to an improved method of, and means for, automatically varying the transmitted audio band Width in response to the amplitude level variation of audio input energy.

Communication systems for the transmission, or reproduction, of audible sound are usually limited in volume range by background noise. It is well known that the annoyance caused by an audible fluctuation noise is greater when the noise contains the higher frequencies of the audible spectrum. This is true because, although the noise energy is uniformly distributed over the frequency spectrum, the useful sound energy is mainly concentrated in the lower frequencies. Thus, the highest effective signal to noise ratio in the presence of disturbing noise is usually obtained by cutting down the highest frequency which can be transmitted. Accordingly, the scratch noise present during playing of an ordinary phonograph record is less annoying when the reproduced audio frequency band does not extend above substantially 3,000 cycles; In high fidelity musical reproduction from records, this difliculty can only be overcome by recording at a sufficiently high level so that the weakest passages of music are suflicient to override the noise. This is not always possible, however, particularly when the music, has loud passages which it is Accordingly, it may be stated that it is one of the main objects of my invention to provide an improved method of, and means for, automatically controlling the band width of the transmitted audio signals by a special regulating device whose function it is to adjust the fremission channel is capable when the level of desired sound energy is above a predetermined value which is sufficiently great substantially to -eliminate annoyance from background noise;

desired to reproduce without over-cutting the and the control additionally reduced the frequency spectrum to an extent sufficient such that undesired noises become less objectionable in the output of the gain transmission channel as soon as the desired sound amplitude level falls below a certain minimum and background noises becomes noticeably great.

Still other objects of this invention are to improve the operation of automatic arrangements for regulating the high audio frequency response of an audio signal transmission system,

the control arrangement including an audio signal rectifier which functionsto varythe gain of an electron discharge tube having circuits asprove generally automatic audio frequency refrequency width of the audio system is in use for high fidelity reproduction. However, as soon as the desired sound energy decreases substantially to noise level, the width of the audio frequency spectrum should automatically be reduced to a value giving a higher effective signal to noise sponse control circuits, and more especially to provide an audio signal transmission system with means for regulating the high audio response thereof, and which control means is reliable in operation and is economically assembled in an audio transmission system.

The novel features which I believe to be characteristic-of my invention are set forth in particularity in the appended claims: the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organiza condenser l l transmits pairof anodes.

reproducing phonograph records, or it may be the detector output circuit of a radio receiver of the'broadcast type. It can even be the microphone of a public address system, or the photoelectric-pick -up device used with talking moving pictures. In general, then, the source I is to be understood as feeding to the potentiometer re sistor 2'audio signal voltage covering a range of approximately zero to 15,000 cycles, which .latter range is generally considered as being the high fidelity audio reproduction range.

The numeral 4 denotes an audioamplifier tube mnemoni or type. For example, it may be the. ei ec 1 trical pick-up device used in connection with veloped across resistor i9 is impressed on the inputpgrifaipf the control tube 22. The control voltage is inipressed on grid 2| through a path which includes the filter resistor 23 and the grid leak resistor 24, and the junction of the resistors is grounded througha condenser 25. f The resistors i9 and Hand condensers 20 and 25 cooperate to provide a resistance-capacitance filter,

of any well known type, and whose signalinput grid 5 is connected by source of audio voltage I. v The cathode-oi tube '4 is connected to groundthroughthe usual resistor-condenser grid biasing network 6.v The screen grid of tube 4 has'positive voltage applied 'to' it' through the usual resistor I.the latter'being by-passed to the cathode-bya capacitor. The

the adjustable tap 3 to the" potentiometer resistor 2 shunted across the ible a full waverectifier is employed time constant needed is only half that which would be required with a half wave rectifier. .Of course, halt wave rectification can be used in' tential for plate and the'time constant ofthis filter should be Just long enough to prevent low frequency alternating voltage from being applied to the grid 2|. In order to make the deviceoperate as rapidly as posso that the such cases where cost considerations make a slight sacrifice in performance desirable. The tube 22 has circuits associated therewith such that the tube functions as an electronic capacitance. Theplate 26 of the control tube is connected to thelplate end of resistor '9 through a blocking .condenser 21, while the cathode of the tube is connected to ground through the resistor 28. The positive po- 26 is applied through a path which includes the coil L and resistor R. A capacitance 29 is connected between grid 2| and the junction of resistor R and coil L, and the capaciusual self-biasing tance'iunctions as a blocking condenser. Grid leak resistor 24 is a high resistance resistor.

'- Considering the control. tube22 more specifically suppressor grid-of tube 4 is at groundpotential,

while the plate 8 is established at adesired positive potential by the load resistor 9.. Numeral ll designates schematically a succeeding audio amplifier network, and the latter-may comprise one or more audio amplifier tubes. The coupling The amplified audio voltageoutput oi amplifier I!) may be impressed upon any desired utilization means, suchv as a reprpducer or the loudspeaker type. The high audio frequency response oi the audio response characteristic is automatically varied by the control channel which includes an amplifier l2. It is to be understood that this tubes, or, if desired,

\ the audio voltage devel- 'oped across'load resistor Sto the input electrode it is to be noted that the' gri'd 2| 'is' connected to plate 26 through the relatively high resistanceR. while inductance 1 connects thecontrol grid to the cathode: If an alternating voltageis applied across the plate to cathode path, and if resistor R is sui'flciently high, a. current will flow through and L which is substantially in phase with the applied voltage. The voltage drop across coil L is applied to the currentand thusthe applied voltage) by approximately 90 degrees.

Theresulting alternating current plate current due to this grid voltage will then lead the applied voltage by about 90 degrees. Assuming R is so amplifier may include one or more amplifier the amplifier-may be omitted.

A desired value of audio signal voltage is im- 9 pressed on amplifier 12 by providing in shunt with resistor 2 a resistor l3. The adjustable tap 5 l4 enables the operator to select the desiredv ma nitude of audio signal voltage which is to be employed for the control function. In other words, taps 3 and provide independent means for permitting the regulation or the magnitudes of the input audio. signal voltage and the control audio voltage. This is o! decided advantage in providing increased flexibility of control. The output'of amplifier I2 is impressed-upon a full wave rectifier which comprises. the double diode tube I5 having a commoncathode and a One of the anodes I5 is connected to one end of the secondary winding of 7 audio transformer", while the anode I8 is connected to the opposite end of the. said winding. The grounded cathode of tube I5 is connected to the mid-point of the secondary winding through a load resistor l9 which is appropriately by-passed by condenser 20. The uni-directional voltage dehigh that the current through it can be neglected, the alternating" voltage applied between plate and cathode of tube 22 results in a current across'these electrodes which leads the voltage by approximately degrees. In other words, the impedance between the plate and cathode of tube 22 functions in the manner of a capacitance, and

the magnitude of this capacitance is proportional to the alternating plate current. Thus, when 1 the .transconductance of tube 22 is varied as by varying the bias ofgrid2l, the eflective capacitance between the" cathode and plate is varied.

- It can be demonstrated that a-variation or the Fur of tube 22 will vary the effective capacitance provided by the impedance between the'cathode and plate of tube 221almost linearly,- thus justifying the concept that the'tube22 behaves as a variable capacitance. v I v -To"use tube 22 for variable high frequency audio cut-oii the plate to cathode impedance of the tube is connected across the resistive plate load 9 of amplifiergtube 4. In Fig. 2 there is shown various frequency response curves secured with different values of transcpnductance for tube 22. In securingthe various curves in Fig. 2 it was assumed that the resistive plate load of the amplifier 4 is of the order of 100,000 ohms, and in vconsideringthis magnitude the plate resistances of tubes4 and 22am to be included in thisyalue.

grid'2l and is then leading the The resistor R had a value of 200,000 ohms, while coil L had a value of 0.5 henry; it was considered that coil L had internal losses corresponding to a series resistor value of 200 ohms, and that am of tube 22 had a value of 2X mhos maximum. An inspection of the curves of Fig. 2 will show that the Irequency response is varied from the uppermost fiat curve, with a value of cm which equals zero, to the lowermost of! around 1,000 cycles. Also, it will be observed that the high audio frequency response depends directly on the transconductance of the tube 22. The control grid bias of tube 22 is varied without the consumption of power, and, therefore, represents a desirable manner of controlling the frequency response of the audio signal transmission channel. a c

In considering the operation of the circuit arrangement shown in Fig. '1, the audio input voltage 'isapplied at the. adjustable potentiometer 2+4 and the amplified, audio voltage is transmitted to the utilization means as described above. The frequency response of the main transmission channel will be determined by the response characteristic of plate load resistorv 9 and the shunt admittance connected across it by means of blocking condenser 21 and tube 22 and its associated components. The audio input signal is simultaneously applied to the adjustable potentiometer I2l| which may be'adjusted for selecting thescratch amplitude level which is decurve which cuts tion shown by Fig. 2 is accompanied by a very slight volume expansion action. For low levels of the incoming sound, the relative maximum gain is slightly less than for high levels of the incoming sound. This behaviour is caused by the internal losses in the coil L together with the fact that the resistor -R is not infinitely larger than the reactance of L. As a result the tube 22 and its accompanying circuit does not behave as a pure variable capacitance, but it also behaves as a variable conductance. The circuit values which were used for the illustrative curves ,of Fig. 2 were chosen, for simplicity, so that the volume expansion action was very small. By increasing L and decreasing R, or by inserting additional resistance in series with L, the volume expansion action .may be made appreciable.

Such behaviour is of advantage when the volume range of the incoming sound has been restricted.

since it tends to restore the original volume range.- The same action also tends to minimize sired to be tolerated. After implification atamplifier l2 and rectification at double diode l5,

the control voltage is applied to the grid 2| of the tube 22. a

With noises, such as scratch noises, only impressed at the input network of the system, tap I0 is adiusted upwardly along resistor i3 until tube 22'just begins to decrease in plate current.

Under these conditions tube 22 is operated at approximately maximum transconductance, and

the response. curve is similar to one of the lower curves shown in Fig. 2. The scratch components in the audio output of amplifier iii are, therefore, less annoying because of the absence of high audio frequency components. When the audio signal amplitude exceeds the scratch, or noise, level slightly, the control grid 21 is biased more negatively and the response curve of the system approaches one of the higher curves of Fig. 2. For audio signal amplitudes well above the scratch level audio reproduction is in accordance with the uppermost curve of Fig. 2, since in that case tube 22 is completely cut-off. In this event full high fidelity reproduction is available. It will, therefore, be seen that the audio control channel provides a means for utilizing a unidirectional voltage derived irom the audio signal itself, and which uni-directional voltage is employed to vary the gain oi a tube whose plate to cathode'impedance functions as a capacitance connected in shunt across the main audio signal transmission channel.

In some cases it may be advantageous to use a delayed bias on the double diode It. Again, it is not essential to the present invention that the input to the control channel be derived from across source i, since the input to the control channel may be taken from the plate resistor 9 or even from across the output of amplifier 10. Additionally, the variable electronic capacitance may be connected at other points of the transmission channel for varying the response characteristic.

nois'e'and scratcli'which occur at low sound levels since not only is the frequency range reduced by the shunting capacitance action of tube 22, but the gainat all frequencies may be reduced by the shunt conductance action of the same tube.

While I' have indicated and described a system for carrying my invention into efl'ect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In an audio transmission system of the type including at least one audio amplifier tube and a utilizing means, a frequency response control arrangement including an electron discharge tube having circuits associated therewith so as to render the plate to cathode impedance of the tube operable as a reactive impedance, means for connecting said reactive impedance across the output of said amplifier tube, and means connected across the amplifier tube input for deriving a uni-directional control voltage from audio voltage impressed upon said amplifier tube, and means for impressing said uni-directional voltage upon a control electrode of said control tube thereby to vary the magnitude of said reactive impedance.

2. In combination with a main audio voltage transmission channel, a control tube having its plate to cathode impedance connected across the channel to function as a shunt reactive path, and means connected across the main channel input and directly responsive to amplitude of the audio signal voltage impressed upon said main channel for controlling the gain of said control tube thereby to control the magnitude of said shunt reactive path.

3. In an audio voltage transmission system ofand control grid, a connection between the out-' put electrode of the control tube and said audio transmission system for impressing audio volt- It should, also, be pointed out that the operaage between the cathode and output electrode

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